The invention disclosed herein is generally related to photometric instruments and, more specifically, to instruments for measuring the intensity of Cerenkov radiation in water-filled nuclear fuel storage ponds.
Nuclear reactor fuel is ordinarily contained in what are known as nuclear fuel assemblies. Such fuel assemblies are the basic fuel units which are loaded into a nuclear reactor core and removed at a later date when the nuclear fuel is spent. A typical fuel assembly consists of a bundle of up to several hundred parallel fuel rods. Each fuel rod consists of a metal tube loaded with nuclear reactor fuel. The fuel rods are spaced from one another to permit circulation of coolant water through the fuel assembly, and also in accordance with design considerations related to the power output of the reactor.
When a fuel assembly is removed from the reactor core at the end of a fuel cycle, the spent fuel is highly radioactive and continues to produce a substantial amount of heat and radiation for a number of months. Accordingly, the fuel assemblies are stored in water-filled storage ponds until such time as the spent fuel can be safely reprocessed or permanently disposed of. However, both reprocessing and permanent disposal of spent nuclear fuel are, for the most part, presently being held in abeyance pending the resolution of complex technical and political issues. As a result, large quantities of spent nuclear fuel are accumulating in storage ponds throughout the world, and will continue to accumulate in the forseeable future.
The spent nuclear fuel includes both fissile material and nonfissile fission products. The fissile material consists primarily of unburned fissile fuel, mostly U.sup.235 ; and fissile Pu.sup.239, which is formed by neutron capture in fertile U.sup.238 originally present in the nuclear fuel. Both U.sup.235 and Pu.sup.239 are special nuclear materials and as such are potential targets of attempted wrongful diversion for the purpose of making nuclear weapons. To deter attempts at such diversion, and to discover it after the fact in the event it is accomplished, there exists a cooperative international program to periodically inspect nuclear fuel assemblies in storage throughout the world and to maintain an accounting of the spent nuclear fuel contained in such assemblies.
Diversion of spent nuclear fuel could be, for example, by outright theft of a fuel assembly. This type of diversion is readily detectable. A more difficult problem in terms of detection is posed by more sophisticated types of diversion wherein the removal of spent fuel might be concealed in some manner. For example, an irradiated fuel assembly might be removed and replaced with either an empty fuel assembly or a fuel assembly containing a dummy fuel material. Conceivably, the contents of a particular fuel assembly could even be removed and replaced with a dummy material. In view of these possibilities, a method has been sought by which irradiated fuel assemblies in storage ponds may be routinely inspected in situ to determine whether they have been replaced or altered in any way.
Such a method is disclosed in the U.S. patent application Ser. No. 151,870 of Dowdy et al., filed May 21, 1980, which is hereby incorporated by reference. In accordance with the method disclosed therein, a measurement is made of the intensity of Cerenkov radiation produced by a fuel assembly in a storage pond.
Cerenkov radiation appears as a blue glow in the water around an irradiated fuel assembly. It is produced by the interaction between gamma radiation, which is emitted by radioactive fission products in the spent fuel, and the surrounding water. Since the radioactive fission products decay with time, the intensity of both the gamma radiation and the induced Cerenkov radiation progressively decrease with time. The rate at which both types of radiation decreases depends largely on two factors; the length of time the fuel assembly was in the reactor core, and the average power level at which the reactor was operated during such time. As a result, each fuel assembly or set of fuel assemblies associated with a particular fuel cycle has a predictable pattern of progressively decreasing Cerenkov radiation emission, which pattern extends over a period of years after the assembly has been removed from the reactor core. At any time during this period, the actual level of Cerenkov radiation from a given fuel assembly can be measured and compared with the predicted level for the assembly. Thus, the pattern of Cerenkov radiation emission provides a basis for periodically inspecting the fuel assembly to ascertain that it has not been altered or replaced. Moreover, the pattern of Cerenkov radiation emission for each fuel assembly is, to some extent, unique, and thus provides a basis for ascertaining the identity of the fuel assembly.
As discussed more fully in the Dowdy et al. application, a measurement of the Cerenkov radiation intensity of a particular fuel assembly is best taken along vertical coolant channels which are located between the fuel rods and which extend the full length of the fuel assembly. The Cerenkov glow is brightest in these channels and is relatively free of light from nearby fuel assemblies. Also, the glow in the coolant channels, when viewed end on, represents an average, or integrated, intensity for the entire length of the fuel assembly. To take such a measurement thus requires an instrument that can be aligned and focused on the coolant channels where the Cerenkov glow is brightest.
The actual measurement of Cerenkov radiation from a large number of fuel assemblies poses certain other requirements, particularly where it is sought to make such measurements quickly, efficiently, and routinely by an inspector in the field. For example, the large number of fuel assemblies in storage around the world makes it imperative that an instrument for this purpose be portable, reliable and efficient. Also, it is well known that the Cerenkov glow from a fuel assembly diminishes to a level barely perceptible to the unaided eye within a few months or years, so that some form of image intensification may be required to enable a portable instrument to be focused on a region of relatively high Cerenkov radiation intensity where a measurement is to be taken. Further, it is desirable to have a portable instrument which can be focused on a region of maximum Cerenkov radiation intensity, and which can simultaneously obtain a measurement of the Cerenkov radiation intensity while so focused. All of this must be accomplished from a bridge above the storage pond, a position on the order of 30 feet above the tops of the submerged fuel assemblies.
The above-referenced application of Dowdy et al. discloses several types of instruments that have been used to practice the method. However, none of those instruments is portable. More importantly, none of those instruments provides the capability to obtain an intensity measurement at the same time as the instrument is aimed and focused on a particular fuel assembly.
Accordingly, it is the object and purpose of the present invention to provide a portable instrument for inspecting an irradiated nuclear fuel assembly in a water-filled storage pond by measurement of the induced Cerenkov radiation associated with the fuel assembly.
It is also an object and purpose of this invention to provide such an instrument having image intensifying capability, whereby an operator can focus the instrument on a region of highest Cerenkov radiation intensity which may be nevertheless invisible to the unaided eye.
It is a further object of this invention to provide a portable instrument which attains the foregoing objects and purposes, and with which an operator can obtain Cerenkov radiation intensity measurements while simultaneously focusing the instrument on a selected region of highest Cerenkov radiation intensity.